Many types of aircraft are required to be highly maneuverable to perform their functions. Examples include aerobatic light aircraft, trainers, and fighter or attack aircraft. Such aircraft are occasionally required to operate in flight regions where the angle-of-attack is large. Angle-of-attack refers to the incidence of an aircraft with respect to its velocity vector. For aircraft to be able to fly safely at high angle-of-attack and perform maneuvers, the aircraft must be stable and controllable. In some modern aircraft, stability is provided by artificial means. Artificial stabilization may be achieved with control effectors, which are used to generate forces and moments to oppose unwanted aircraft motions. While artificial stabilization may be effective, it has some unfavorable side effects. For example, as the level of instability increases, the amount of control moment required to stabilize the vehicle increases. Beyond a certain level of instability, sufficient control moments may not be available, and the aircraft can experience a departure from controlled flight. Even if sufficient control authority is available to prevent unwanted motions, artificial stabilization results in less control power available to maneuver the aircraft. A loss of available control power results in a loss in mission effectiveness. Artificial stabilization is not an option with most low cost aircraft since such aircraft do not have computers and control effector designs required to add artificial stability. It is noted that a loss in inherent vehicle stability occurs with virtually all aircraft at high angles-of-attack.
While stability in pitch axis can be altered with careful design and control of aircraft center-of-gravity, directional stability must be provided by stabilizing surfaces or by artificial means. Directional stability is defined as the tendency of an aircraft to weathercock into the wind when disturbed. When used without reference to an axis system, directional stability is assumed to be in the flight path or stability axis system. Although other axis systems may be used for convenience, the flight path or stability axis reflects the motion of the aircraft. Flight path directional stability is composed of two parameters, i.e. body axis directional stability and body axis lateral stability or dihedral effect. The body axes of a vehicle are mutually perpendicular and are normally aligned along the fuselage axis, the approximate plane of the wing, and normal to the other two. Body axis directional stability refers to the tendency of the fuselage to point back into the wind when disturbed from equilibrium. Body axis lateral stability refers to a tendency for the aircraft to roll in a direction to eliminate any side component of the relative wind. If an aircraft is at an angle-of-attack other than zero, flight path axis directional stability is a combination of both body axis directional and lateral stability and is calculated from equation 1 as follows: ##EQU1## From equation 1 it is apparent that both body axis directional stability and body axis lateral stability contribute significantly to flight path directional stability when angle-of-attack is large. This implies that aircraft stability can be improved by any device or component that increases either or both values. It is highly desirable to provide inherent directional stability to maximize the amount of control power available to perform out of plane maneuvers, such as rolling, and to prevent rapid aircraft departures from controlled flight, which often lead to spins. Current and prior design practice has relied on large fixed aerodynamic surfaces mounted on the aircraft fuselage to stabilize the aircraft. However, such surfaces lose their effectiveness at large incidence angles.
The current invention relies on a novel arrangement of wing and tail surfaces to provide inherent directional stability even at high angle-of-attack. The invention achieves these results by joining the wing and horizontal tail panels with canted or vertical stabilizing surfaces that operate effectively over a wide angle-of-attack and sideslip range and provide stabilizing moments about both directional and lateral axes. These stabilizing moments will insure that the aircraft remains directionally stable, or nearly so, to much higher angles-of-attack than is current practice.
Directional stability has typically been provided by a single vertical tail located on the centerline of an aircraft at or near the rear of an aircraft fuselage. The vertical tail provides directional stability by acting as a lifting surface. When an aircraft is perturbed in a manner such that sideslip occurs, a local angle-of-attack occurs in the plane of the vertical tail. The local angle-of-attack generates a lifting force on the tail panel and creates a moment about the center-of-gravity of the aircraft that opposes the sideslip and returns the aircraft to a zero sideslip condition. The larger the tail surface area, the larger the moment that is generated, and the greater the directional stability.
Although the use of a vertical tail to provide directional stabilization has proven effective over the years, a vertical tail presents serious disadvantages at high angles-of-attack. As aircraft angle-of-attack increases, the fuselage of the aircraft tends to block airflow to the vertical tail. The blockage of airflow reduces the effectiveness of the vertical tail. If angle-of-attack exceeds a certain value, this value being dependent on the vehicle in question, the flow over the fuselage separates, or detaches. Under a detached flow condition, not only is the tail partially blocked by the fuselage, but flow separation results in a region of low energy air in the vicinity of the vertical tail, which further reduces the effectiveness of the tail.
An additional factor resulting in reduced effectiveness of the tail is the aft sweep of a typically configured tail. High performance aircraft typically employ swept wing and tail surfaces to reduce drag at transonic and supersonic Mach numbers. Due to structural considerations, these surfaces, including the vertical tail panel, are usually swept aft. For a vertical tail, aft sweep results in much of the airflow at high angles-of-attack being directed along the span of the surface of the tail rather than along the chord line of the tail, which further reduces effectiveness of the tail.
A single, centerline vertical tail has additional disadvantages for modern high performance aircraft. Highly swept surfaces at the front of typical modern high performance aircraft are intended to generate vortices, or regions of high energy rotational flow, at high angles-of-attack. These vortices have been shown to interact with downstream aircraft components, sometimes in an unfavorable manner. Using the F-16 aircraft as an example, under certain conditions, the vertical tail actually contributes a destabilizing directional moment.
Several modern high performance fighter aircraft employ twin vertical tails to partially overcome the disadvantages of a single tail surface. For a given aircraft configuration, the total area of the twin panels is usually more than a single panel configured to provide equivalent directional stability at low angles-of-attack. In all known current applications, the twin vertical tails are mounted on the fuselage. Therefore, at high angle-of-attack, the fuselage still tends to block a portion of airflow to the tail surfaces. Additionally, energy of the flow in the vicinity of the tail surfaces is reduced. On aircraft configurations set up to generate strong vortex flow fields, twin tail surfaces can reduce but not eliminate the effect of unfavorable interference due to vortex impingement and interaction. An additional problem encountered by fuselage mounted twin vertical tail configurations is the loss of effectiveness due to sweep back.
Other arrangements have been proposed or used to provide inherent directional stability, but most of these are intended for low performance aircraft operating at low angle-of-attack. Examples of other arrangements include twin horizontal stabilizer mounted vertical surfaces (e.g., the B-24 bomber of World War II), boom mounted tails (e.g., the P-38 fighter aircraft), and a wing tip mounted vertical tail (e.g., the Beechcraft Starship and numerous light plane designs). All of these concepts attempt to improve upon conventional means of stabilization. None are entirely successful. A new approach is required to provide directional stability at high angles-of-attack.
Historical results have shown that aerodynamic advantages exist for the biplane configuration. A biplane consists of two lifting surfaces or panels separated in height and sometimes longitudinal location. Test results have shown that the effective aspect ratio of a biplane is higher than that of a monoplane of the same span. The change in effective aspect ratio has been shown to be a function of main surface wing span, vertical distance between the lifting surface panels, ratio of the span of one panel to that of the other, and relative lengthwise positioning of the two panels, i.e., stagger.
Attempts have been made in modern designs to make use of the benefits of the biplane. Such attempts have largely consisted of designs that join together tips of forward and aft lifting surfaces. These attempts have generally not been successful for a number of reasons. First, the direct joining of the tips of two lifting surfaces results in no vertical separation between the panels at the tips. Test results indicate that a condition of no vertical separation between lifting surfaces eliminates the desired biplane effect. Second, joining of the lifting surfaces limits design options. For example, if the tips of the lifting surfaces are joined, it is usually necessary for both lifting surfaces to have the same span, which results in a tandem wing configuration that is not ideal for all applications.
An additional attempt to make use of the benefits of a biplane is a boxplane configuration. A boxplane configuration joins upper and lower wing panels in a rigid structure. However, a box plane arrangement uses a conventional horizontal stabilizer and does not attempt to use the joining members as vertical stabilizers. Other modern designs have attempted to use a biplane arrangement for supersonic vehicles. These designs may take advantage of favorable shock wave interaction at supersonic Mach numbers. Due to the nature of shock waves and their inclination with respect to an aircraft's direction of motion, a supersonic biplane arrangement is not suitable for a conventional wing and tail arrangement where the separation of wing and tail surfaces is set by considerations of aircraft balance and control capability rather than simply being determined by shock wave inclination at a design Mach number.